1. Field of the Invention
The present invention relates to a speed reducer, and more particularly to a mechanical paradox planetary speed reducer.
2. Description of the Related Art
One mechanical paradox planetary speed reducer disclosed in Japanese laid-open patent publication No. 8-303531 comprises an external sun gear fixedly mounted on an input shaft, a fixed internal sun gear disposed around the external sun gear and nonrotatably fixed to a casing, a plurality of planet gears disposed between and meshing with the external sun gear and the fixed internal sun gear, and a movable internal sun gear rotatably positioned in juxtaposed relation to the fixed internal sun gear in the axial direction of the input shaft and held in mesh with the planet gears. The rotation of the input shaft is transmitted at a speed reduction ratio via the fixed internal sun gear and the planet gears to the movable internal sun gear, from which the rotation is outputted via an output gear to an output shaft.
The disclosed speed reducer is advantageous in that it is small in size, but provides a large speed reduction ratio.
In the conventional mechanical paradox planetary speed reducer disclosed in Japanese laid-open patent publication No. 8-303531, the fixed internal sun gear and the movable internal sun gear are completely separate from each other. Therefore, while the speed reducer is in operation, the fixed internal sun gear and the movable internal sun gear may possibly be displaced out of coaxial alignment with each other. When the fixed internal sun gear and the movable internal sun gear are misaligned, each of the gears is subject to undue stresses, causing the speed reducer to operate unstably and reducing the service life of the speed reducer.
When the conventional mechanical paradox planetary speed reducer is assembled, it is difficult to keep the fixed internal sun gear and the movable internal sun gear in concentric relation to each other. Therefore, it tends to be tedious and time-consuming to bring these internal sun gears into mesh with the planet gears in the process of assembling the conventional mechanical paradox planetary speed reducer.
It is therefore an object of the present invention to provide a speed reducer which allows fixed internal sun gears and a movable internal sun gear to be reliably and easily kept concentric to each other when the speed reducer is in operation or is assembled, so that the speed reducer can operate stably and can be assembled with ease.
To achieve the above object, there is provided in accordance with the present invention a speed reducer comprising an external sun gear fixed to an input shaft, a pair of fixed internal sun gears nonrotatably fixed in position and disposed around the external sun gear, a plurality of planet gears interposed between and held in mesh with the external sun gear and the fixed internal sun gears, and a movable internal sun gear rotatably disposed in juxtaposed relation to the fixed internal sun gears in an axial direction of the input shaft and held in mesh with the planet gear, whereby the rotation of the input shaft can be transmitted at a speed reduction ratio to the movable internal sun gear, the fixed internal sun gears being spaced from each other in the axial direction of the input shaft, the movable internal sun gear being disposed between the fixed internal sun gears, the movable internal sun gear having ends confronting respective ends of the fixed internal sun gears, the ends of the movable internal sun gear and the ends of the fixed internal sun gears being held in sliding contact each other to hold the movable internal sun gear and the fixed internal sun gears in concentric relation to each other.
With the above arrangement, since the movable internal sun gear and the fixed internal sun gears are concentrically held in sliding contact with each other, the movable internal sun gear and the fixed internal sun gears remain stably concentric each other while the speed reducer is in operation. In assembling the speed reducer, the movable internal sun gear and the fixed internal sun gears remain concentric to each other when they are held in fitting engagement with each other. Consequently, they can smoothly be brought into mesh with the planet gears, and hence the speed reducer can smoothly be assembled.
Therefore, the fixed internal sun gears and the movable internal sun gear are reliably and easily held in concentric relation to each other when the speed reducer is in operation or is assembled. The speed reducer can thus operate stably, and can be assembled with ease.
According to a specific engaging structure, one of the ends of the movable internal sun gear and the ends of the fixed internal sun gears which confront the ends of the movable internal sun gear has an outer flange fitted over the other confronting end in sliding contact therewith.
Since the outer flange of one of the ends of the movable internal sun gear and the ends of the fixed internal sun gears is fitted over the other confronting end in sliding contact therewith, one of the movable internal sun gear and the fixed internal sun gears is prevented from being relatively displaced from the others in a direction perpendicular to their axes. Therefore, the movable internal sun gear and the fixed internal sun gears are steadily kept concentric to each other.
According to another specific engaging structure, the ends of the movable internal sun gear have respective slanted surfaces on end faces thereof, and the ends of the fixed internal sun gears which confront the ends of the movable internal sun gear have respective slanted surfaces on end faces thereof, the slanted surfaces being inclined to the axis of the input shaft and held in sliding contact with each other.
The slanted surfaces of the movable internal sun gear and the fixed internal sun gears engage each other to prevent one of the internal sun gears from being relatively displaced from the others in a direction perpendicular to their axes. Therefore, the movable internal sun gear and the fixed internal sun gears are steadily kept concentric to each other.
According to still another specific engaging structure, the speed reducer further includes rolling bearing mechanisms interposed between the ends of the movable internal sun gear and the ends of the fixed internal sun gears which confront the ends of the movable internal sun gear, for assisting the movable internal sun gear in rotating with respect to the fixed internal sun gears.
The rolling bearing mechanisms interposed between the movable internal sun gear and the fixed internal sun gears are effective to prevent one of the internal sun gears from being relatively displaced from the others in a direction perpendicular to their axes. Therefore, the movable internal sun gear and the fixed internal sun gears are steadily kept concentric to each other. The rolling bearing mechanisms allow the movable internal sun gear to rotate smoothly with respect to the fixed internal sun gears.
The above engaging structures may be used in combination in the speed reducer.
Each of the ends of the movable internal sun gear and the ends of the fixed internal sun gears which are held in sliding contact with each other has a self-lubricating coating.
The self-lubricating coating can sufficiently reduce the friction caused by the sliding contact between the ends of the movable internal sun gear and the ends of the fixed internal sun gears, allowing the movable internal sun gear to rotate smoothly.
The self-lubricating coating may comprise a coating of PTFE (polytetrafluoroethylene) or molybdenum disulfide, for example.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.